This application is based upon Japanese Patent Application Nos. Hei. 11-106913 filed on Apr. 14, 1999, and 2000-52163 filed on Feb. 23, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to semiconductor strain sensors, and particularly to a semiconductor strain sensor in which a sensor element (sensor chip) is mounted on a lead frame fixed to a resin package member, and is applicable to a high pressure sensor such as an intake pressure sensor or a brake oil pressure sensor for a vehicle.
2. Related Art
Conventionally, this kind of semiconductor strain sensors has proposed in, for example, JP A 9-61271 or JP A 5-172674. According to these semiconductor strain sensors, a lead frame is molded by a resin package member so that both main and back surfaces of the lead frame are covered with the package member, and a sensor element (sensor chip), which is made of silicon and is capable of converting a strain while external stress (pressure or acceleration) is applied into an electric signal is mounted on the lead frame by using resin adhesive.
In a case of pressure detection, for example, when pressure (e.g., intake pressure of a combustion engine or brake oil pressure for a vehicle) is applied to the sensor element from a target medium to be measured, the sensor element strains so that an electric signal in proportion to the strain (hereinafter, called xe2x80x9cstrain signalxe2x80x9d) is outputted from the sensor element. The strain signal outputted from the sensor element is externally outputted from a lead frame through a bonding wire.
In the conventional structure, thermal expansion coefficients of the sensor element, the lead frame and a package set are set to similar values for the purpose of reducing thermal stress to be applied to the sensor element to secure a sensor characteristic. However, thermal hysteresis, in which the sensor characteristic changes from an initial characteristic as a result of a cooling-heating cycle that repeats low temperatures and high temperatures, will occur even in such a structure.
FIG. 7 shows a diagram illustrating the thermal hysteresis. In this figure, a horizontal axis (shown by dotted line) indicates time, a left vertical axis indicates sensor outputs (strain signal) (shown by solid line), and a right vertical axis indicates device temperatures.
The thermal hysteresis is a difference between former sensor outputs at a room temperature (e.g., 25xc2x0 C.) and latter sensor outputs at the room temperature after passing a high temperature process (e.g., 120xc2x0 C.). In other words, the thermal hysteresis is a phenomenon in which the sensor outputs at the room temperature after passing the high process are changed with time.
According to the consideration, this phenomenon occurs due to the following reason. An initial stress while the lead frame is being molded with a package member or a difference of thermal expansion coefficients between each member occurs as a contortion as a result of the thermal process. A creep is undergone in the resin, which constitutes the package member. In the sensor element after passing the thermal process, stress (hereinafter, called xe2x80x9ccreep stressxe2x80x9d) due to the creep deformation of the package member is applied to the sensor element, unlike an initial condition before the thermal process. Therefore, an output (strain signal) characteristic of the sensor element changes, and therefore the thermal hysteresis occurs.
Furthermore, in a case where the sensor element and the lead frame are fixed to each other via resin adhesive, the creep stress of the package member is also applied to the adhesive, and creep also occurs in the resin constituting the adhesive. The creep stress in the adhesive also affects the sensor element.
This problem that a sensor characteristic changes due to a creep stress, which is undergone in a resin constituting a package, is a common problem for semiconductor strain sensors in which a sensor element for detecting a strain signal is mounted in a resin package member.
This invention has been conceived in view of the background thus far described and its first object is to prevent creep stress of a package member from affecting a sensor element.
Its second object is to provide a semiconductor strain sensor in which a sensor element for detecting a strain signal is mounted in a resin package member and which can prevent creep stress from the package member from affecting the sensor element.
According to the present invention, a lead frame has one surface for mounting a sensor element and another surface, and is supported by a package member. The package member has a space portion on a side of the another surface of the lead frame. The space portion is formed so that at least one area of a mounting area where the sensor element is mounted in the another surface of the lead frame is non-contacted with the package member at the space portion. Due to the existence of the space portion, even if the creep occurs in the package member, creep stress is prevented from being transferred to the sensor element.
According to another aspect of the present invention, a sensor element is contained in a resin package member via an element mounting member, which has a thermal expansion coefficient closer to that of the sensor element than that of the resin constituting the package member. Due to the existence of the element mounting member having such a characteristic, thermal stress due to a difference in the thermal expansion coefficient between the sensor element and the package member can be prevented, and therefore creep in the package member can be prevented. As a result, creep stress in the package member can be prevented from affecting to the sensor element.